Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes the location and orientation of the wellbore and drilling assembly, earth formation properties, and drilling environmental parameters downhole. Directional information relating to surveying the location of the wellbore, and controlling or "steering" the drilling assembly, will be discussed later.
The collection of information relating to formation properties and conditions downhole, commonly referred to as "logging," can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a driller regarding the particular earth formation being drilled. In conventional wireline logging, a probe or "sonde" housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde is supported by a conductive wireline, which attaches to the sonde at the upper end. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
More recently, the industry has placed an increased emphasis on the collection of data during the drilling process itself. By selecting and processing data during the drilling process, without the necessity of removing or tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections on-the-fly, as necessary, to optimize performance. Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
Ordinarily, a well is drilled vertically for at least a portion of its depth. The layers or strata that make up the earth's crust are generally substantially horizontal. Therefore, during vertical drilling, the well is substantially normal to the geological formations through which it passes One of the properties of the formation that is commonly logged is its resistivity. LWD tools that are designed to measure the resistivity of the surrounding formation need not be azimuthally focused, as the formation in question surrounds the wellbore and is essentially the same in all directions. Thus the rotation of the LWD tool with the bit has no significant effect on the measured resistivity. For this reason, typical LWD resistivity tools that are adapted for use in vertical wells are azimuthally symmetric and have no azimuthal sensitivity.
In certain applications, however, such as when drilling from an off-shore platform, or when drilling through formations in which the reservoir boundaries extend vertically, it is desirable to drill wells that are oriented more horizontally. When drilling horizontally, it is desirable to maintain the well bore in the pay zone (the formation which contains hydrocarbons) as much as possible so as to maximize the recovery. Formations, however, may dip or divert, making it difficult to stay within the boundaries of the desired formation. Thus, while attempting to drill and maintain the well bore within a particular formation, the drill bit may approach a bed boundary. As the rotating bit approaches the bed boundary, the bed boundary will be on one side of the bit axis, or in one azimuthal range with respect to the bit axis.
If a near-bit resistivity tool were capable of sensing resistivity values azimuthally, the sensed values could be analyzed to discern the direction of the bed boundary. If the tool were sufficiently sensitive, the approaching bed boundary would be detected in sufficient time to allow the driller to make corrections in accordance with known techniques to avoid exiting the desired formation.
Various devices have been proposed for measuring resistivity azimuthally. For example, U.S. Pat. No. 4,786,874 describes a resistivity measuring tool which uses an asymmetrical generation of current in the formation by a current electrode placed on one side of the drill collar and the sensing of asymmetrical voltage distribution in the formation by a voltage sensing electrode, with both electrodes placed on an insulated section of the drill collar. Another tool for providing azimuthally sensitivity resistivity measurements is set out and described in U.S. Pat. No. 5,045,795. This tool provides a pair of toroids on which four coils are mounted. The two toroids are connected by magnetic shorting bars. The coil segments and shorting bars inscribe a specific solid angle or azimuth. By connecting the outputs of the several coils through a combining circuit, the coils on a single coil supporting form can be connected in a series, additive or subtractive relationship. Through the use of two such coil forms with the line coils on each, an azimuthally oriented window is thereby defined.
Neither of these prior art systems, however, will work in synthetic drilling muds or oil based muds (non-conductive muds). Hence, it is desirable to provide an LWD tool that allows azimuthally sensitive resistivity measurements, that is easy to manufacture and assemble and that is sufficiently durable and reliable in the drilling environment.